In optical communication systems which carry out optical communications by using optical fiber lines, it is important to detect failures such as breaks in the optical fiber lines and increases in transmission loss. In particular, when a failure occurs in an optical fiber line or subscriber terminal in subscriber optical communication systems which have been becoming widespread in recent years, it is required for them to rapidly specify the position of failure and repair the failure.
Therefore, the optical communication systems are provided with optical line monitoring apparatus for detecting such failures. The optical line monitoring apparatus, which utilize optical reflectometry techniques, determine a reflectance distribution in an optical line such as an optical fiber line according to reflected light (constituted by Fresnel reflected light and Rayleigh scattering light) occurring when monitor light propagates through the optical line, thereby detecting a position of failure in the optical line. It has been required for such optical line monitoring apparatus to measure the reflectance distribution with a high spatial resolution.
Known as an optical reflectometry technique is OTDR (Optical Time Domain Reflectometry) which measures a reflectance distribution according to temporal changes in intensity of reflected light occurring when pulsed monitor light propagates through an optical line. As another optical reflectometry technique, OCDR (Optical Coherence Domain Reflectometry) has been known (see Patent Literature 1 and Non Patent Literatures 1 and 2).
The OCDR generates monitor light whose optical frequency is modulated such as to yield a comb-teeth-shaped optical coherence function, inputs reflected light occurring when the monitor light propagates through an optical line, inputs reference light taken out as a branch of the monitor light, and measures a reflectance at a specific position of the optical line by utilizing the fact that the magnitude of interference between the reflected light and reference light depends on the delay time difference therebetween. Further, while changing positions for measuring the reflectance by altering a period of optical frequency modulation in the monitor light and so forth, the OCDR determines a reflectance distribution in the optical line.
The optical coherence function is obtained by normalizing an autocorrelation function <V(t)·V*(t−τ)> of an electric field V(t) of light, which is a function including time t as a variable, with optical intensity, i.e., by normalizing a Fourier transform of an optical power spectrum with optical intensity. Assuming that light having an electric field V(t) is split into two and that the delay time difference between the resulting two branches of light is τ, the magnitude of interference fringes between the two branches of light is represented by the real part of the optical coherence function. The absolute value of the optical coherence function is also known as degree of coherence and indicates the magnitude of interference.
The monitor light used in the OCDR, which is one whose optical frequency is modulated, has a comb-teeth-shaped optical coherence function. In a specific example, light whose optical frequency is modulated at fixed time intervals into f0, f0+fs, f0−fs, f0+2fs, f0−2fs, f0+3fs, f0−3fs, . . . in this order is employed as the monitor light. In another example, light whose optical frequency is modulated into a sine wave at a modulation frequency of fs is used as the monitor light. The optical coherence function of the monitor light whose optical frequency is thus modulated has a peak (coherence peak) shaped similar to a delta function when fsτ is an integer. Hence, these kinds of monitor light have a comb-teeth-shaped coherence function. The position of the coherence peak varies as fs changes.
The comb-teeth-shaped coherence function has a plurality of coherence peaks arranged at a period of 1/fs. A gate having a time width shorter than the period (1/fs) of coherence peaks is applied to the monitor light such that one of the coherence peaks exists in a segment to be measured in an optical line, whereby a pulse of the monitor light is cut out.